wchar_t mdash[] = { 0x2014, 0x0000 };

std::wfstream os("filename.txt", std::ios_base::out| std::ios_base::trunc);
os << mdash;
assert(!os.bad()); // fails

void set_locale_on_stream(std::wfstream &os)
{
    char* locale = setlocale(LC_ALL, "English"); // Get the CRT's current locale.
    std::locale lollocale(locale);
    setlocale(LC_ALL, locale); // Restore the CRT.
    os.imbue(lollocale); // Now set the std::wcout to have the locale that we got from the CRT.
}

int foo()
{
    wchar_t mdash[] = L"— 😃 Test";
    const wchar_t *filename = L"filename.txt";

    wchar_t wbuf[128];
    std::wofstream fout(filename, std::ios::binary);
    if(fout)
    {
        fout.rdbuf()->pubsetbuf(wbuf, 128);

        //optional BOM
        wchar_t bom[1] = { 0xFEFF };
        int x = sizeof(bom);
        fout.write(bom, 1);

        fout << mdash;
        fout.close();
    }

    std::wifstream fin(filename, std::ios::binary);
    if(fin)
    {
        fin.rdbuf()->pubsetbuf(wbuf, 128);

        //optional, skip BOM
        std::wstring wstr;
        if(fin >> wstr)
            MessageBoxW(0, wstr.c_str(), 0, 0);
        fin.close();
    }
    return 0;
}